The present invention generally relates to an electrographic process and more particularly, to a method of controlling replenishment of toners of a developing material accommodated in a developing material tank of an electrostatic latent image developing device for use, for example, in an electrographic copying apparatus and the like.
Commonly, in the development of electrostatic latent images, it has been known that developed images of an extremely high quality may be obtained if a developing material containing two kinds of toners which can be triboelectrically charged to polarities opposite to each other is employed. Moreover, by employing such a developing material as referred to above, it becomes possible to readily change over between a regular development (positive-positive development) and a reversal development (negative-negative development), and also to effect a two-color development. However, in order to effect a stable development by the use of the developing material as described above, it is necessary to properly maintain the toner concentration of the developing material accommodated in a developing material tank of a developing device, and more specifically, the mixing ratio of respective toners contained in the developing material.
In connection with the above, for properly controlling the mixing ratio of the respecitve toners as described above, it is preliminarily required to accurately detect the concentration (mixing ratio) of the respective toners in the developing material accommodated within the developing material tank of the developing device, and subsequently, to replenish the developing material tank with a fresh toner until the concentration (mixing ratio) of the respective toners to be detected based on the result of the above detection reaches a proper reference or standard value.
Conventionally, for the toner concentration detecting method related to the so-called dual-component or two-component magnetic developing material composed of a mixture of an electrically insulative toner and a magnetic carrier, there have been known various detecting methods for measuring the magnetic variation, capacity variation or variation of electrical resistance values of the developing material arising from variation in the toner concentration, through utilization of differences in the physical properties between said toner and carrier. However, in the detecting method as described above, since variations in the physical properties of the developing material on the whole due to the variation of the toner concentration are measured, it is not possible to accurately detect the concentration (mixing ratio) of the respective toners in the developing material containing two kinds of toners resembling each other in physical properties, so that there is a consequent difficulty in effecting the control for properly maintaining the mixing ratio of the respective toners.